1. Field of the Invention
This invention relates to a novel process for preparing cysteamine-S-substituted compounds and derivatives thereof. More particularly, the invention relates to a very useful and advantageous pollution-free process for preparing said compounds with high purity in a high yield, said process requiring only one step and involving no possibility of producing any harmful substance.
2. Description of the Prior Art
Cysteamine-S-substituted compounds and derivatives thereof are known as substances having a radiation protective activity. They are also very useful as intermediates for medicinal preparations.
For the preparation of said compounds, there has been generally employed a process according to which, as shown by the following formulae, an ethyleneimine derivative is ring-opened with hydrobromic acid to form a bromoalkylamine hydrobromide which then is reacted with a thiosulfate or thiophosphate.
(H. Bretschneider: Monatsh. Chem. 81, 372 (1950); S. Akerfeldt: Acta Chem. Scand. 13, 1479 (1959)). ##STR1##
This known process, however, is very poor in yield: the overall yield from ethyleneimine is as low as 33.3% in the case of cysteamine-S-sulfuric acid (2-aminoethanethiosulfuric acid) and only 42.8% in the case of sodium monohydrogen cysteamine-S-phosphate. Further, as obvious from the above-shown reaction formulae, sodium bromide is formed as by-product at a rate of 2 moles to one mole of the objective material and, moreover, the objective material is readily soluble in water and shows the same behavior as the by-product sodium bromide, so that separation of the by-product sodium bromide is extremely difficult and its contamination in the final product is unavoidable. Thus, it is hardly possible with this process to obtain the objective substance with high purity.
Furthermore, according to the conventional process, the reaction can not be completed in one stage and needs to be carried out in two stages. In addition, it is essential in the first-stage reaction to use hydrobromic acid at a rate of 2 moles to one mole of ethyleneimine, said hydrobromic acid being a substance which, when exposed to air, produces bromine to give rise to the problem of environmental pollution and insanitation. To make the matter worse, the conventional process requires use of a large amount of this substance which is very expensive. Also, use of hydrobromic acid inevitably causes coloring of the products.